Elemental copper and copper-containing metal oxides are widely employed in industry and see frequent use particularly in the fields of co-catalysts and antibacterial agents. Applications as co-catalysts have included, in recent years, use for the visible-light sensitization of photocatalysts, in asymmetry catalysts for organic synthesis, in catalysts for the atom transfer radical polymerization (ATRP) of polymeric compounds, and in steam-reforming catalysts for various types of organic compounds. Also, it has long been known that elemental copper and copper ions have antibacterial properties, and the mechanisms that lead up to bacteriostasis and bacterial death via enzyme inhibition, membrane protein denaturation and structural protein denaturation in cell organelles of the target bacteria have continued to be studied even recently. Copper-containing catalysts produced with these effects in mind are manufactured, for example, by adding copper ions to a carrier having cation exchangeability and immobilizing the copper ions by ion exchange, or by supporting copper oxide in the form of fine particles on a carrier (JP-A H06-065012: Patent Document 1; JP-A H11-349423: Patent Document 2; JP-A 2007-131551: Patent Document 3), and are used in a wide variety of hygiene products. However, in all of these copper-containing metal oxides, the copper composition changes readily depending on factors in the surrounding environment, such as light, heat and moisture. Hence, while such catalyst sensitizing effects and antibacterial activities can all be expected to be high initially, they rapidly deteriorate over about several weeks, resulting in a loss of efficacy that presents a problem in applications where such effects are required to persist over a long period of time.
Titanium oxide is used in various applications, including pigments, UV blockers, catalysts, photocatalysts, catalyst carriers, adsorbents, ion-exchange agents, fillers, reinforcing agents, starting materials for ceramics, as a precursor for complex oxides such as perovskite-type complex oxides, and in undercoats for magnetic tape.
Of these, photocatalytic titanium oxide fine particles are a type of material having a photocatalytic action when irradiated with light that includes ultraviolet light at wavelengths of up to 400 nm. As used herein, “photocatalytic action” refers to an action where holes and electrons that have been generated by excitation with UV light at up to 400 nm and have diffused to a surface carry out redox reaction together with molecules adsorbed to that surface. Organic matter adsorbed to the titanium oxide surface is decomposed by these redox reactions. By coating titanium oxide fine particles having this photocatalytic action onto a substrate surface so as to form a photocatalytic thin-film and irradiating the coated surface with excitation light, harmful organic matter adsorbed to the surface can be decomposed. Hence, titanium oxide fine particles are widely used in applications such as the cleaning, deodorization and disinfection of substrate surfaces. Titanium oxide fine particles are often used in combination with copper compounds to increase such antibacterial activity and other properties (JP-A H08-067835: Patent Document 4). Composite materials of copper compounds and titanium oxide catalysts are especially promising in applications where antibacterial properties are required.
To increase the photocatalytic activity, there is a desire that the contact surface area between the photocatalyst particles and the substance to be decomposed be made larger. Satisfying this desire requires that the average particle size of the titanium oxide fine particles within the titanium oxide dispersion be 50 nm or less.
Common methods for producing titanium oxide fine particles include industrial processes such as the sulfate process and chloride process which use ilmenite ore or rutile ore as the raw material (Sanka chitan [Titanium oxide], published by Gihodo Shuppan: Non-Patent Document 1). Other methods include a hydrolysis-calcination process, reaction in an organic solvent, and a solid phase process (Hikari-syokubai hyoujun kenkyuho [Standard research methods for photocatalysts], published by Tokyo Tosho: Non-Patent Document 2). These titanium oxide fine particles are coated onto a substrate surface. To retain the decorative qualities of the substrate to be coated, ultrafine dispersion treatment in a liquid coating is carried out. Examples of common fine dispersion treatment methods include methods which use a wet dispersing machine and a dispersing agent to disperse within a dispersion medium a finely divided titanium oxide powder that has been synthesized (JP-A H01-003020: Patent Document 5; JP-A H06-279725: Patent Document 6; JP-A H07-247119: Patent Document 7; JP-A 2004-182558: Patent Document 8), and methods which stably disperse titanium oxide fine particles within a dispersion medium by surface treating the titanium oxide (JP-A 2005-170687: Patent Document 9; JP-A 2009-179497: Patent Document 10). However, one problem with these methods is that because ultrafine particles having an average particle size of 50 nm or less have a tendency to agglomerate, considerable effort is required to achieve a dispersion down to the primary particles; in some cases dispersion down to the primary particles is impossible. Another problem is that the particles are surface-treated with inorganic or organic ingredients and dispersing agents such as surfactants are added in order to increase the dispersion stability; however, the photocatalyst surface ends up coated with these substances, which interferes with the appearance of a photocatalytic activity.
In addition, a method for producing an anatase-type titanium oxide dispersion having long-term stability that entails the hydrothermal treatment of a peroxotitanic solution obtained by dissolving titanium hydroxide in hydrogen peroxide (JP-A H10-067516: Patent Document 11), a method for producing a rutile-type titanium oxide sol (JP-A H02-255532: Patent Document 12), and a method for producing a titanium oxide sol (JP-A H10-182152: Patent Document 13) have been disclosed. In these dispersions, the titanium oxide fine particles are dispersed to an average particle size of 50 nm or less without using surface treatment or a dispersing agent, and photocatalytic coating films obtained by coating such dispersions onto a substrate have an excellent transparency and exhibit activity under UV irradiation. Although these titanium oxides do exhibit a good photocatalytic activity when irradiated with the ultraviolet region light having a relatively short wavelength (10 to 400 nm) included in, for example, sunlight, it is sometimes difficult for sufficient photocatalytic activity to appear within indoor spaces illuminated with light sources that supply for the most part light in the visible region (wavelength, 400 to 800 nm) such as fluorescent lamps.
One visible light-sensitive photocatalyst capable of obtaining sufficient photocatalytic effects even within indoor spaces that is worthy of note is a tungsten oxide photocatalyst (JP-A 2009-148700: Patent Document 14). In addition, a tungsten oxide photocatalyst with a copper compound supported on the surface has been disclosed (JP-A 2009-226299: Patent Document 15).